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. 2016 Apr 1;53(2):511-518.
doi: 10.1111/1365-2664.12575. Epub 2015 Dec 18.

An introduced parasitic fly may lead to local extinction of Darwin's finch populations

Jennifer A H Koop  1 , Peter S Kim  2 , Sarah A Knutie  1 , Fred Adler  3 , Dale H Clayton  1
Affiliations

An introduced parasitic fly may lead to local extinction of Darwin's finch populations

Jennifer A H Koop et al. J Appl Ecol. .

Abstract

Introduced pathogens and other parasites are often implicated in host population level declines and extinctions. However, such claims are rarely supported by rigorous real-time data. Indeed, the threat of introduced parasites often goes unnoticed until after host populations have declined severely. The recent introduction of the parasitic nest fly, Philornis downsi, to the Galápagos Islands provides an opportunity to monitor the current impact of an invasive parasite on endemic land bird populations, including Darwin's finches.In this paper we present a population viability model to explore the potential long-term effect of P. downsi on Darwin's finch populations. The goal of our study was to determine whether P. downsi has the potential to drive host populations to extinction and whether management efforts are likely to be effective.Our model is based on data from five years of experimental field work documenting the effect of P. downsi on the reproductive success of medium ground finch Geospiza fortis populations on Santa Cruz Island. Under two of the three scenarios tested, the model predicted medium ground finches are at risk of extinction within the next century.However, sensitivity analyses reveal that even a modest reduction in the prevalence of the parasite could improve the stability of finch populations. We discuss the practicality of several management options aimed at achieving this goal.Synthesis and applications. Our study demonstrates the predicted high risk of local extinction of an abundant host species, the medium ground finch Geospiza fortis due to an introduced parasite, Philornis downsi. However, our study further suggests that careful management practices aimed at reducing parasite prevalence have the potential to significantly lower the risk of host species extinction.

Keywords: Galápagos; Geospiza fortis; Philornis downsi; conservation; invasive species; medium ground finch; model; parasite; population viability model.

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Figures

Figure 1
Figure 1
Diagram of model predicting finch annual reproductive success. The variable An is the number of female adults in the population at the beginning of year n, which is equal to the carrying capacity K in our model. The model accounts for the following processes: (1) Each breeding season, a certain fraction of female adults breed (pn) and produce a mean number of clutches per adult (cn) for a cumulative number of Cn clutches in a breeding season. (2) In a given year, a fraction (qn) of clutches will become infested (In) by P. downsi or remain uninfested (Un). (3) Each infested (mn,rnI) and uninfested (mn,rnU)) clutch successfully produces a certain number of female fledglings (Fn) in a given year. Each clutch is capable of producing a maximum number (mni) of nestlings and rnIandrnU refer to the probabilities that female nestlings fledge from infested and uninfested nests respectively. (4) A fraction of female fledglings (snF) from year n survive to become adults at the beginning of year n + 1. (5) A fraction of female adults (snA) from year n survive to year n + 1.
Figure 2
Figure 2
Distribution of predicted extinction times over 1000 model simulations for medium ground finches on Santa Cruz Island. Parameters are estimated from data collected over five years of experimental study in the same population of medium ground finches (Table 2). The three scenarios correspond to the three scenarios described in the Materials and methods section and presented in Table 3.
Figure 3
Figure 3
Sensitivity of medium ground finch populations to changes in probability of infestation by P. downsi. Parameter estimates are shown in Table 2 and estimates are based on the model scenario of all years being weighed equally. (a) Mean extinction times (± standard deviation) over 1000 simulations as a function of percentage decrease in parasite infestation. Only the results for decreases of up to 40% are shown. When infestation levels are reduced by more than 40%, simulated extinction times exceed 1000 years. (b) Annual finch population growth rate r as a function of percentage decrease in infestation.
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